It is known that liquid crystal displays with a single domain in each cell or pixel have unsymmetrical viewing angles behavior, especially in the vertical viewing-axis for single-domain LCD's orientated in the conventional manner.
In the current fabrication of single-domain liquid crystal displays, each surface of the panel's transparent electrodes in contact with the liquid crystal (LC) layer is formed with an alignment film. These two films or layers usually are rubbed so as to form the alignment direction as illustrated by the direction of the molecules 28 in FIG. 2. The alignment layers cause the alignment of the longitudinal axes of the liquid crystal molecules and a tilting of the molecules at a small angle, known at the pre-tilt angle, relative to the place of the alignment layer. The pre-tilt angle of the LC molecules adjacent to the alignment films are such that the bulk liquid crystal molecules are caused to take a common orientation with the molecules adjacent the alignment layers so that each pixel or cell will exhibit a single domain when voltage is applied.
Depending on the rubbing directions of the alignment films in the fabrication of the panel and with the display in the conventional orientation as shown in FIG. 1, either the top or bottom viewing zone will have a large reverse contrast whereas the opposite vertical zone (bottom or top) will have poor contrast between gray scale levels.
It has been found that, with the display in the conventional orientation, the vertical viewing angle behavior of the liquid crystal display is improved when the display cells or pixels are each fabricated such that two domains are formed when voltage is applied across the liquid crystal layer. Five methods have been proposed for fabricating such two domain pixels, namely, 1) multiple rubbing as disclosed in JP 63-106624; 2) multiple alignment layer as reported in SID 92 Digest, p. 798; 3) edge field fringe as described in U.S. Pat. No. 5,249,070; 4) parallel fringe as described in U.S. Pat. No. 5,309,264, and assigned to the same assignee as the present application; and 5) UV method as disclosed in U.S. Pat. No. 5,623,354, and assigned to the same assignee as the present application, disclosures of which are incorporated herein by reference.
In method 1), alignment layers in a pixel are each divided by a boundary and the area on one side of the boundary is rubbed in one direction and the area on the other side is rubbed in the opposite direction to form an alignment layer with two areas each with a different alignment direction. When one area is being rubbed, the other area must be protected thereby complicating the fabrication process. Method 2) also is complicated since it requires additional lithographic and etching steps, as well as, deposition of both inorganic and organic alignment layers. An inorganic alignment material is first deposited on each of the transparent electrodes followed by the deposition of an organic alignment on the inorganic layer. A photoresist is coated on the organic layer and patterned and developed so that half of each of the inorganic/organic layers of each pixel are still covered by the photoresist after development. The organic alignment layer in the half where the photoresist has been removed is etched away to leave that half of the pixel with the inorganic alignment layer and the other half with the organic layer after the photoresist is removed. Finally, each of the inorganic/organic layers are rubbed in the appropriate direction to complete the fabrication of the alignment layers, each with two areas. In method 3), only a single domain is present when voltage is not applied, but the fringe fields at opposite edges of the pixel cause the liquid crystal molecules to tilt toward the field at their closest edge when voltage is applied to produce two domains in the pixel. In method 4) a slot is formed or etched in the common electrode in the center of each pixel so that, when voltage is applied, two pairs of parallel fringe fields are produced, one field of a pair by the pixel edge and the other field of this pair by the edge of the slot. These two parallel fringe fields with voltage applied will cause LC molecules on each side of the slot in the pixel to tilt in the direction of the edge fringe field on the respective side of the slot, thereby forming two domains. In method 5), selected areas of alignment layer are exposed to the UV light to change the pre-tilt angle at those areas.
Three of these methods increase the complexity of fabricating a two-domain display panel as compared to a panel with only a single domain per cell. The multiple rubbing method 1) requires protecting part of the alignment layer with photoresist and multiple rubbing of the alignment layer. Similarly, the multiple alignment layer of method 2) and the parallel fringe field of method 4) demand patterning with photoresist and removal of part of the organic layer in method 2) or part of the electrode layer in method 4). Since these three methods necessitate coating, baking, patterning, developing and stripping a photoresist, the fabrication of the display panel by any one of these methods is more complicated and thus increases the manufacturing cost above the cost of a single-domain display panel. Method 3) does not form two areas in each pixel to form a two-domain pixel but, by utilizing the edge fringe fields, creates a two-domain pixel when voltage is applied. However, with this method, the boundary between the two domains is not well defined. Method 5) is simple and effective but it usually requires large UV dosage to create the appropriate pre-tilt angle difference. Due to the high dosage, chain scissioning occurs in the alignment layers, which most commonly are polyimides. Chain scissioning in turn results in free-radical formation and possible charged species. As a result, "image sticking" is observed in the LCD panel fabricated with this method. Image sticking is currently one of the most serious problems which hinders the fabrication of multidomain LCDs with wide viewing angle.